Rime and Graupel

Under some atmospheric conditions, forming and descending snow crystals may encounter and pass through
atmospheric supercooled cloud droplets. These droplets, which have a diameter of about 10 µm, can exist
in the unfrozen state down to temperatures near -40° C. Contact between the snow crystal and the supercooled
droplets results in freezing of the liquid droplets onto the surface of the crystals. This process of crystal
growth is know as accretion. Crystals that exhibit frozen droplets on their surfaces are referred to as rimed.
When this process continues so that the shape of the original snow crystal is no longer identifiable, the resulting
crystal is referred to as graupel. The frozen droplets on the surface of rimed crystals are hard to resolve and the
topography of a graupel particle is not easy to record with a light microscope because of the limited resolution and
depth of field in the instrument. However, observations of snow crystals with a low temperature LT-SEM clearly show
cloud droplets measuring up to 50 µm on the surface of the crystals. The rime has been observed on all four basic forms
of snow crystals, including plates Fig.1 (05219), dendrites Fig.2 (12495),
columns Fig.3 (13371) and needles Fig.4 (23541).
As the riming process continues, the mass of frozen, accumulated cloud droplets obscures the identity of the original snow crystal
thereby giving rise to a graupel particle Fig.5 (17570).